I have no idea what a RTBS is. Tiger barbs are extremely aggressive. I had them and they even picked on the most aggressive fish I had before their addition. They would likely kill your betta and the gourami.

I cannot condone the mixing of fish temperaments. You can do what you feel is right, but I cannot say that it is ok. When adding fish you want to be careful not to add too many at once as it will offset your beneficial bacteria ratio.